This invention relates to multi-port switches and, in particular, to multi-port switches which utilize transmission lines and switching elements to achieve selective coupling between the various switch ports.
In radio frequency (RF) transmission systems, it is customary to utilize some type of multi-port switch to effect coupling amongst the antenna, transmitter and receiver of the system. In certain multi-port switches used for this purpose, switching elements and transmission lines are employed to form the switch. U.S. Pat. Nos. 4,803,447 and 4,701,724 disclose switches of this type.
The '724 patent discloses a switch (FIG. 2), therein in which a quarter-wave length of transmission line is used to connect an antenna to a transmitter and to a receiver. A first PIN diode poled in the direction of the quarter-wave line is situated between a DC blocking capacitor coupled to the transmitter and a first end of the quarter-wave line. A second PIN diode poled in a direction away from the quarter-wave line is connected to a second end of the line and to ground.
The antenna of the '724 system is also connected to the first end of the quarter-wave line and to a blocking capacitor connected to the transmitter. An RF choke is responsive at a first end to a DC bias or control signal and at its second end is connected to the junction of the first PIN diode and the DC blocking capacitor coupled to the transmitter.
With this switch configuration in the '724 patent, a high signal level applied to the first end of the RF choke places both the first and second PIN diodes into conduction. The conduction of the second PIN diode, grounds the second end of the quarter-wave line, making the second end of the line appear as an open circuit to signals at its first end. The conduction of the second PIN diode also grounds the receiver. The receiver is thus decoupled from the antenna, while the transmitter is coupled to the antenna through the first PIN diode. Hence, the system is placed in a transmit mode in which signals can pass from the transmitter to the antenna.
With a low signal level applied to the first end of the RF choke, both the first and second PIN diodes are non-conductive. The transmitter is thereby decoupled by the open circuited first PIN diode from the antenna. The second end of the quarter-wave line is, furthermore, no longer grounded because the second PIN diode also appears as an open circuit. As a result, the quarter-wave line now couples the receiver to the antenna and the system is placed in the receive mode of operation.
The '724 patent also discloses a further switch configuration (FIGS. 3 and 4), therein in which a directional coupler comprised of first and second coupled quarter-wave lines is used. In this case, the first quarter-wave line of the coupler is situated between the antenna and receiver of the system. First and second PIN diodes, in turn, are connected to the first and second ends of the second quarter-wave line. The transmitter is connected to this second end of the second line and control signals are applied to the anode of the first diode to operate the switch.
In the transmission system of the '724 patent, filters are also used with the disclosed switches to filter the received and transmitted signals in a variety of ways. These filters are separate components from the switches, thereby adding both to the cost and size of the overall system.
It is therefore a primary object of the present invention to provide a multi-port switch of the above type which is adapted to reduce the cost of additional filtering components.
It is a further object of the present invention to provide a multi-port switch of the above type which is further adapted to provide filtering as well as switching.